1. Field of the Invention
The present invention relates to the field of cathode ray tube drivers. More specifically, the present invention relates to the field of cathode ray tub driver circuits with cathode frequency compensation and cathode current detection for high resolution display applications.
2. Related Art
Digital television formats such as high definition television (HDTV) and enhanced definition television (EDTV) provides for increased resolution in the television picture. The improvements in picture quality is accomplished via a new broadcasting standard that implements a wider bandwidth and higher frequencies than previously required for use with conventional television units. As such, supporting circuitry in televisions must be adapted to perform under the new broadcasting standard, such as the one supporting HDTV.
One component needing to adapt to the higher bandwidth and higher frequencies of the digital television formats is the cathode ray tube (CRT) driver used for amplifying a video input signal and generating a video output signal for driving a cathode in a television CRT.
In the past, CRT drivers for conventional televisions consisted of discrete circuits capable of easily handling the high power supplies necessary to run conventional CRTs. The discrete circuits also could easily handle the lower frequencies and lower bandwidths of the analog broadcasting signal. However, the discrete circuits could not handle the higher operating frequencies, higher speeds, and wider bandwidths of high resolution televisions. These high resolution televisions (e.g., HDTV and EDTV) are capable of displaying digitally broadcasted signals. As such, new CRT drivers implemented on integrated circuit boards have been developed that are capable of handling the higher operating frequencies, higher speeds, and wider bandwidths required by the high resolution televisions.
In addition, it is desirous for the new CRT driver circuits implemented on an integrated circuit to have cathode current detection. Over time, the cathode in the electron gun of the CRT will deteriorate. This deterioration leads to a fluctuation in the cathode current which leads to a deterioration in color on the television display. The ability to measure the cathode current allows for recalibration of the electron gun in the CRT so that the magnitude of current generated by the cathode, in response to a test input signal, is consistent over time.
Furthermore, it is desirous to speed up the transient response, or the slew rate, of the amplifier contained in the CRT driver circuit. Frequency compensation is one method for increasing speed. By including frequency compensation, the speed of the CRT driver circuit during the transient response periods is markedly increased without increasing power consumption.
Prior Art FIG. 1 shows a typical high frequency amplifier circuit 100 with frequency response using a feedback path consisting of a bypass capacitor 110 and resistor 120. Although circuit 100 provides for increased transient response, the discrete components (e.g. the capacitor 110 and resistor 120) required in circuit 100 are incapable of handling the higher operating frequencies, higher speeds, and wider bandwidths typically required by the high resolution televisions. Furthermore, because of their size and structure, the discrete components cannot be implemented in an integrated circuit, as is typically required by high resolution televisions.
Further, CRT driver circuits implemented on integrated circuits in the prior art that included frequency compensation were also incapable of accurately measuring the cathode current from a CRT that was driven by the CRT driver circuit. For example, frequency compensation circuits implemented on integrated circuits typically provided for a leakage path that allowed a cathode current to leak to ground through the frequency compensation circuit. To compound matters, the leakage current would be of the same magnitude as the cathode current. As such, the CRT cathode current being leaked could not be ignored, and any measurement of the cathode current would not reflect the current lost through the leakage path. Thus, any measurement would not be a true measurement of the cathode current coming from the CRT.
Thus, there is a need for CRT driver circuits that can accurately measure cathode current from a CRT with frequency compensation capabilities.
Accordingly, a cathode ray tube (CRT) driver circuit suitable for high resolution display applications with frequency compensation is described. Additionally, the present invention provides the above accomplishment and is further capable of accurately measuring cathode current from the CRT.
Specifically, the present invention discloses a CRT driver circuit comprising an input buffer stage, a cascode gain stage, as well as a cascode push-pull output stage that is biased as a Class B configuration for driving a CRT cathode. At an output node, the cascode push-pull output stage generates a video output signal in response to an input voltage for driving a coupled CRT and corresponding CRT cathode.
A biasing node within the push-pull output stage has a biasing voltage that is always higher than the output voltage of the video output signal. A frequency compensation circuit is comprised of a vertical-integrated PNP transistor that is adaptively coupled to the biasing node, the output node, and the cascode gain stage.
Electrodes of the vertical-integrated PNP bi-polar junction transistor (BJT) are adaptively coupled to the biasing node, output node, and the cascode gain stage to provide two parallel feedback paths for frequency compensation. In addition, the N-P junction capacitances within the PNP BJT are reversed biased so that any cathode current leakage through the PNP BJT is effectively eliminated. As such, a current detection circuit accurately measures cathode current from a CRT without any loss in cathode current through the frequency compensation circuit.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.